旋转流化床粉体混合机混合效果数值模拟和实验验证

为了对旋转流化床粉体混合机进行优化设计,采用CFD-DEM联合仿真的方法,对旋转流化床粉体混合机内球形颗粒的混合过程进行数值模拟,通过Lacey指数具体评价颗粒的混合效果,研究了进气管倾斜角度、进气管布置方式、进气方式对球形颗粒混合效果的影响,并进行球形颗粒混合实验验证。结果表明,进气管最合适的倾斜角度应保证气流作用区域面积恰好为底部颗粒物料区域面积的一半。进气管水平布置时能够保证很好的混合质量及较快的混合速率。脉冲及连续方式进气均能实现均匀混合,脉冲进气方式比连续进气方式耗气量更低。颗粒混合实验有很好的混合效果,与数值模拟的结果具有较高的一致性,从而获得了一种混合效果优越的结构形式,进气管倾斜角度α=35°,水平布置。     

基于CFD的泵送混凝土流变性能数值模拟研究

预测新拌混凝土在泵管中的流变性复杂且困难,而计算流体力学(CFD)提供了一种模拟泵送混凝土流变性能的新方法,具有预测结果准确且可视化、减少试验工作量等优点.为此通过采用CFD中的Fluent数值模拟软件模拟了C60新拌混凝土在盘管试验中的流变性,重点分析了具有不同厚度润滑层的混凝土在泵管中流速、压力分布以及泵送压力损失的变化规律,通过与试验结果进行了对比分析,提出了模拟新拌混凝土泵送性的建模方法,结果表明,当润滑层厚度为6 mm左右时,模拟结果与实测结果吻合良好,验证了数值模拟方法的有效性.     

离心流化床初始流化状态的研究(Ⅱ)实验研究

对离心流化床初始流化状态进行了实验研究。结果表明,下行程床层压降流速曲线的规律一致,流化时床层压降趋于稳定;上行程曲线受床体结构、物料装填方式和运行状况的影响很大。实验测定初始流化速度应采用下行程曲线。可视性实验证实了离心流化床逐层初始流化,已流化部分的床层物料充分混合。     

基于高阶块体元-有限元建模的混凝土细观数值分析

提出一个高阶块体元-有限元的混合建模方法进行混凝土的细观数值模拟,即采用高阶块体元离散骨料区域、有限元离散水泥砂浆区域、高阶块体元与有限元相结合的界面元离散混凝土内部的界面过渡区,既避免了复杂的网格划分及其带来的庞大计算规模问题,又便于边界条件的施加。在传统塑性损伤模型的框架下,建立了一个关于微裂纹表征的水泥砂浆损伤模型,该损伤模型能够反映由微裂纹产生所导致泊松效应弱化现象;提出一个界面过渡区的双曲滑移开裂面,并通过相对开裂位移量和相对滑移位移量,定义了界面抗拉强度和内聚力的后继软化规律,同时引入二次流动势面反映压应力状态下界面过渡区的非关联流动特性;采用建立的高阶块体元-有限元混合模型对混凝土进行细观非线性计算分析,获得了混凝土内部界面过渡区和砂浆的破坏特点,揭示了骨料对损伤路径的影响规律。     

新拌混凝土可泵性的研究进展EI北大核心CSCD

综述了新拌混凝土的可泵性的近期理论研究成果。介绍了新拌混凝土的稳定流动状态,分析了对新拌混凝土与泵管壁之间的润滑层和剪切滑移层的状态与作用。讨论了新拌混凝土的流变性能。除对新拌混凝土的流变模型进行分析外,还描述了新拌混凝土在泵送前后的流变性能的差异。对目前较为认可的新拌混凝土可泵性评估模型和测试方法作了评述。     

浇注方式对超高性能纤维增强混凝土中纤维取向及分布的影响

通过改变新拌浆体的浇筑方式(单侧浇筑和混乱浇筑)以及流动距离(400 mm,600mm,800 mm以及1000 mm),研究超高性能纤维增强混凝土(UHPFRC)中纤维取向与分布.结果 表明:采用从模具单侧浇注的方式以及增长流动距离都可改善纤维的取向与分布.此外,新拌UHPFRC中纤维的流动过程可分为三个区域:混乱期,稳定期和再混乱期.     

混凝土中邻近集料表面最近间距分布的计算机模拟

混凝土中邻近集料问界面过渡区的相互影响程度、混凝土中原生裂纹的尺度范围以及中心质假说中集料效应圈的范围等都涉及到混凝土中最邻近集料表面间距分布的问题。由于常规的实验方法无法给出混凝土中集料空间分布的信息,同时以往的计算机模拟方法由于采用随机分布方式分布粒子,导致无法得到较高集料体积分数的模型混凝土结构,而采用具有粒子动态混合密实功能的SPACE系统,模拟了高集料体积分数混凝土的结构。在假定模型混凝土中集料的最小粒径为1mm的前提下,以符合Fuller分布为例,研究了模型混凝土中集料粒径分布和集料体积分数对邻近集料表面最近间距分布的影响。     

双层涡轮桨搅拌反应器内混合时间的大涡模拟

采用计算流体力学(CFD)方法对直径为0.476m双层涡轮桨搅拌反应器内的流动及混合进行了数值模拟,并实验测试了混合过程。利用大涡模拟(LES)及Smagorinsky-Lilly亚格子模型求解湍流流动与示踪剂传递过程,桨叶区域采用滑移网格技术。研究结果表明,大涡模拟得到的示踪剂响应曲线和混合时间与实验结果吻合良好,其预测精度明显优于基于雷诺平均(Reynolds-averaged Navier-Stokes,RANS)的标准k-ε模型的模拟结果。大涡模拟是研究搅拌反应器内非稳态及周期性湍流流动的有效方法。     

新拌混凝土可泵性的研究进展

综述了新拌混凝土的可泵性的近期理论研究成果。介绍了新拌混凝土的稳定流动状态,分析了对新拌混凝土与泵管壁之间的润滑层和剪切滑移层的状态与作用。讨论了新拌混凝土的流变性能。除对新拌混凝土的流变模型进行分析外,还描述了新拌混凝土在泵送前后的流变性能的差异。对目前较为认可的新拌混凝土可泵性评估模型和测试方法作了评述。     

十字形挡板造粒室内干法制粉混料过程数值模拟

为改善干法制粉造粒室内颗粒混合特性,使其具有良好的造粒效果.采用计算流体力学方法,构建欧拉-欧拉双流体模型,对气固两相流场进行数值模拟,分析了十字形挡板造粒室内的压力场、速度场及颗粒体积分数,并结合实验与数值结果进行对比.结果表明:底部加装十字形挡板后,造粒室底部的压力分布状况更为均匀,提高了造粒室内颗粒整体速度;十字形挡板造粒室内颗粒体积分布达到71％,颗粒能够充满造粒室大部分区域,改善了底部颗粒堆积现象;颗粒合格率提高了6.9％,流动性指数提高了6;实验结果验证了数值模拟的正确性,十字形挡板造粒室有利于促进颗粒混合,提高了颗粒的合格率及流动性,造粒效果更优.     

The effect of interparticle cohesion on powder mixing in a ribbon mixer

The effect of interparticle cohesion on powder mixing in a ribbon mixer was studied by means of the discrete element method. It is shown that with an increase in the cohesion, the mixing rate and uniformity of mixing deteriorate, the coordination number increases indicating the loss of the ability of particles to be engaged in free flowing motion, and a majority of particles have a stronger tangential velocity allowing bulk angular motion of particles. Conversely, with a decrease in the cohesion, more particles have larger axial velocities, which will increase convective motion in the axial direction. When the cohesion is reduced, the number of particles having large radial stresses increases, and normal stress in the axial direction remains mostly unchanged. The ribbon mixer can mix cohesive particles in a wide range of the Bond numbers without causing large stresses. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1023–1037, 2016     

数值模拟静态混合器结构对PS/CO_2熔体温度的影响

利用专用CFD软件Polyflow对SMX型和Kenics型静态混合器中PS/CO_2发泡溶液进行数值模拟计算,分析比较不同板厚在不同元件个数条件下两种静态混合器消耗的压力损失,以及不同CO_2浓度对静态混合器压力损失的影响;并引入"离散系数"分析比较两种静态混合器出口温度均匀性的变化.数值模拟的结果表明:SMX型静态混合器冷却效果优于Kenics型静态混合器,并且SMX型静态混合器出口温度均匀性高于Kenics型静态混合器.     

双排叶片数目对搅拌质量影响的研究

为了探究合理数目的双排叶片对搅拌质量的影响,根据双卧轴搅拌机的本身结构特点。运用离散元分析软件EDEM,仿真物料在拌筒内运动的过程。结果表明:在搅拌质量和搅拌效率方面,一半双排叶片数目的搅拌装置和全部双排叶片数目的搅拌装置仿真效果基本一致。     

Effects of rotational speed and fill level on particle mixing in a stirred tank with different impellers

The particle mixing was studied in a cylindrical stirred tank with elliptical dished bottom by experiments and simulations.The impeller types used were double helical ribbon(HR) + bottom HR,pitched blade ribbon + bottom HR,inner and outer HR + bottom HR,and pitched blade ribbon + Pfaudler + bottom HR labeled as impellers Ⅰ to Ⅳ,respectively.The quantitative correlations among the rotational speed,fill level and power consumption for impeller Ⅰ and impeller Ⅱ were obtained by experiments to validate the discrete element method(DEM) simulations.The particle mixing at different operating conditions was simulated via DEM simulations to calculate the mixing index using the Lacey method,which is a statistical method to provide a mathematical understanding of the mixing state in a binary mixture.The simulation results reveal that as the rotational speed increases,the final mixing index increases,and as the fill level increases,the final mixing index decreases.At the same operating conditions,impeller Ⅲ is the optimal combination,which provides the highest mixing index at the same revolutions.     

基于离散元方法的差动自清洁混合器混合特性研究

采用离散元方法对LIST卧式差动双轴搅拌设备内高密度聚乙烯（PE?HD）填充碳酸钙复合材料的混合过程进行了仿真模拟，研究了不同加料速率及不同转速条件对该类自清洁混合器混合腔内物料充满度、物料停留时间分布及混合均匀程度的影响。结果表明，该类自清洁混合器的轴向输送能力主要由后续物料的推动作用实现，采用高速轴转速40 r/min，加料速率为10 kg/h的工艺参数可以获得最佳的混合均匀程度。     

Effects of retempering on consistency and compressive strength of concrete subjected to prolonged mixing

In this study, effects of prolonged mixing and four different retempering processes on some properties of fresh and hardened concrete, such as temperature, slump loss, and strength, were investigated. Two types of concrete mixtures with different compression strength having 15 cm initial slump were produced in a laboratory mixer. After mixing for 5 min at 20 rpm speed to ensure homogeneity, the mixing was continued at 4 rpm for a period of up to 4 h to simulate the prolonged agitation of ready-mixed concrete in truck mixers. Concrete samples were taken out of the mixer at the end of first, second, third, and fourth hour for estimating the effects of prolonged mixing on properties of fresh concrete. For restoring the initial workability, four different retempering methods were used and their effects on properties of concrete were investigated. Results show that compared to the untempered concrete mixtures, those tempered with solutions prepared by 3% or 4.5% solid superplasticizer by mass of retempering water had significantly less loss of 28-day compressive strength.     

Kenics混合器混合性能的模拟研究

利用Fluent有限元分析软件计算了流体流过Kenics混合器过程中的应变速率,进而分析混合元件转速与旋转式Kenics混合器混合效率的关系,以及混合元件与机筒间隙对静态和旋转式Kenics混合器对混合效率的影响。模拟分析结果表明:旋转式Kenics混合器混合效率随转速增加而提高;减小混合元件与机筒间间隙有利于增加静态Kenics混合器混合效率,但间隙的减小对旋转式Kenics混合器混合效率的影响却很小。     

Numerical analysis of agitation torque and particle motion in a high shear mixer

Numerical simulation of particle motion in a high shear mixer was conducted using a three-dimensional discrete element method (DEM). Torque of an agitator blade, particle velocity profiles and forces acting on a particle were calculated under various agitator rotational speeds. The agitation torque showed a good correlation with the agitator rotational speed and reflected the particle motion in the mixer. The simulated results revealed that particles received greater forces near the bottom of the vessel, as compared to the upper or middle height in the mixer. The particle flow changed with the agitator rotational speed, as experimentally observed, and indicated different velocity profiles. Particle kinetic energy determined the agitation torque and the particle behavior, regardless of the complexity of the changes in the particle flow. A significance of monitoring of the agitation torque in the mixer is also discussed here from the perspective of the particle kinetic energy.     

Distributive mixing in conveying elements of a ZSK-53 co-rotating twin screw extruder

A fluid dynamics analysis package (FIDAP), using the finite element method, was implemented to simulate the 3-D isothermal flow patterns in the conveying element of a ZSK-53 co-rotating twin screw extruder. The fluid was described by a power-law model. The dynamics of distributive mixing was studied numerically by tracking the motion of particles in the mixer. The extent of distributive mixing was characterized in terms of length and area stretch as well as strain distributions. The length stretch reflects the overall capability of the mixing device to spread minor component particles away from their neighbors originally present in the same cluster. The area stretch reflects the evolution of intermaterial area when mixing two fluids with a passive interface. We observed an oscillatory behavior for the average intermaterial area stretch, which was explained in terms of a stretching and folding mechanism. Folding occurs during material takeover from one screw to the other. Operating at higher rotational speeds enhances distributive mixing efficiency.     

粉体混合过程及搅拌功率的DEM数值模拟和实验

采用数值模拟与功率测试相结合的方法,研究直叶桨式粉体混合机搅拌过程及搅拌功率、扭矩的变化规律。对粉体混合机内球形颗粒的混合过程进行离散单元法DEM数值模拟,研究直叶桨式粉体混合机内搅拌转速、搅拌桨直径、桨叶数目等特性参数对粉体混合时搅拌功率和扭矩的影响,并拟合得到功率计算公式。搭建粉体搅拌试验台,测试粉体搅拌功率并与模拟结果比较。结果表明,直叶桨式粉体混合机内功率消耗与搅拌桨转速、搅拌桨直径、桨叶数目等特性参数有密切关系。同时,扭矩值和功率值与搅拌桨转速、搅拌桨直径和桨叶数目都呈正相关。实验得到了与模拟类似的扭矩-转速关系以及功率-转速关系,模拟值与测试值具有较好的吻合性,验证了所推导公式的准确性。